1. Field of the Invention
This invention relates to a silicon nitride sintered body having excellent properties such as toughness and strength, and more specifically, relates to a silicon nitride sintered body used as automotive parts such as piston pins or engine valves, as gas turbine engine parts and as a drive shaft of a fuel jetting pump shaft, a plunger, a washer, a container for a pulverizing machine, a lining material and a media of communication.
2. Description of the Prior Art
It is known that a silicon nitride sintered body has excellent properties such as heat resistance, thermal shock resistance, and oxidation resistance, and investigations as engineering ceramics used in heat engines such as a turborotor have been conducted.
As such silicon nitride sintered bodies, a ternary-type sintered body composed of Si.sub.3 N.sub.4 -rare earth element oxide-SiO.sub.2 is known. In this sintered body, various crystal phases having excellent high temperature characteristics are precipitated in a grain boundary. Accordingly, this sintered body has excellent strength at a high temperature and excellent oxidation resistance at a high temperature. However, to obtain this ternary-type sintered body, it is necessary to fire the starting material at a high temperature of at least 1900.degree. C. while suppressing the decomposition of Si.sub.3 N.sub.4, and moreover, it is necessary to use a special and expensive firing furnace. As a result, the sintered body has a defect that the production cost is very high.
To increase the sinterability, it is also known to add rare earth element oxides such as Y.sub.2 O.sub.3 or oxides such as Al.sub.2 O.sub.3 and MgO to a silicon nitride powder and to fire this mixed powder. Especially when a rare earth oxide and Al.sub.2 O.sub.3 are used, since they can be fired at a low temperature and under a normal pressure, the sintered body can be formed at a low cost. However, when Al.sub.2 O.sub.3 is used, a low melting point compound is easily formed in a grain boundary of silicon nitride crystal. Accordingly, there is a defect that the resulting silicon nitride sintered body has a low strength at a high temperature.
The resulting silicon nitride sintered body has a high strength, but its toughness is as low as 5 MPa.m.sup.1/2, and its toughness should desirably be increased. As a method of increasing the toughness, it has been proposed that the atmosphere or the temperature at the time of firing is controlled to controll the shape of a columnar crystal composed cf silicon nitride, or particles or a fibrous product composed of silicon carbide is mixed with a starting powder to obtain the sintered body which has a complex structure. However, since in the method of controlling the shape of a silicon nitride crystal by the firing atmosphere or the firing temperature, the firing conditions change in accordance with the characteristics of the starting material, it is difficult to produce a sintered body having fixed properties stably, and there is a problem of mass producibility. Furthermore, a method of using a compound of silicon carbide makes it possible to increase toughness to some degree, but excellent characteristics which a silicon nitride sintered body possesses are impaired. Furthermore, sinterability will be lowered and it is necessary to improve sinterability.